The local.config of overlay+tmpfs is as follows:
```local.config.example
export FSTYP=tmpfs
export TEST_DEV=tmpfs_test
export TEST_DIR=/tmp/test
export SCRATCH_DEV=tmpfs_scratch
export SCRATCH_MNT=/tmp/scratch
```
Run `./check -overlay tests` to execute test case on overlay+tmpfs.
Signed-off-by: Baokun Li <libaokun1@huawei.com> Reviewed-by: Amir Goldstein <amir73il@gmail.com> Signed-off-by: Zorro Lang <zlang@kernel.org>